Combustible gases are handled in many industrial applications, including utilities, chemical and petrochemical manufacturing plants, petroleum refineries, metallurgical industries, distilleries, paint and varnish manufacturing, marine operations, printing, semiconductor manufacturing, pharmaceutical manufacturing, and aerosol can filling operations, as a raw material, product or byproduct. In addition, combustible gases are released by leakage from above- or below-ground piping systems, spillage of flammable liquids, or decomposition of natural organic material in the soil or sanitary land fills.
A combustible gas is any gas or vapor that can deflagrate in response to an ignition source when the combustible gas is present in sufficient concentrations by volume with oxygen. Deflagration is typically caused by the negative heat of formation of the combustible gas. Combustible gases generally deflagrate at concentrations above the lower explosive limit and below the upper explosive limit of the combustible gas.
In a deflagration, the combustion of a combustible gas, or other combustible substance, initiates a chemical reaction that propagates outward by transferring heat and/or free radicals to adjacent molecules of the combustible gas. A free radical is any reactive group of atoms containing unpaired electrons, such as OH, H, and CH.sub.3. The transfer of heat and/or free radicals ignites the adjacent molecules. In this manner, the deflagration propagates or expands outward through the combustible gas generally at velocities from about 0.2 ft/sec to about 20 ft/sec. The heat generated by the deflagration generally causes a rapid pressure increase in confined areas.
To reduce the likelihood that a deflagration will occur, regulations often require deflagration suppression systems in the above-noted applications. Deflagration suppression systems generally include a sensor to detect the occurrence of a deflagration and a device to inject a deflagration suppressant into the combustible gas when a deflagration occurs.
The most widely used deflagration suppressants are saturated chlorofluorocarbons, such as Halon 1301 (bromotrifluoromethane), Halon 2402 (dibromotetrafluoroethane) and Halon 1211 (bromochlorodifluoromethane). The saturated chlorofluorocarbon can be injected into the combustible gas either as a vapor or liquid. Due to the low boiling point and low heat of vaporization of saturated chlorofluorocarbons (e.g., the boiling point is typically no more than about 0.degree. C. and the heat of vaporization no more than about 100 cal/g), liquid chlorofluorocarbons will in most applications immediately vaporize upon injection into the combustible gas.
After injection, the saturated chlorofluorocarbon vapor not only dilutes the oxygen available for the combustion of the combustible gas but also impairs the ability of free radicals to propagate the deflagration. The dilution of the oxygen decreases the concentration of the oxygen available to react with the combustible gas and thereby slows the propagation rate of the deflagration. The saturated chlorofluorocarbon vapor impairs the ability of free radicals to propagate the deflagration by reacting with the free radicals released in the combustion reaction before the free radicals can react with combustible gas molecules adjacent to the deflagration.
The use of saturated chlorofluorocarbons has recently been curtailed in response to the environmental hazards associated with saturated chlorofluorocarbon emissions. Specifically, saturated chlorofluorocarbon emissions have a high atmospheric ozone depletion potential and are believed to contribute to the depletion of the ozone layer in the earth's upper atmosphere. Several nations have recently enacted legislation restricting the use of saturated chlorofluorocarbons. Additionally, a large number of nations have recently become parties to an international accord to ban the production of saturated chlorofluorocarbons.
In addition to the environmental hazards of saturated chlorofluorocarbons, byproducts of the reaction of saturated and unsaturated chlorofluorocarbons and combustible gas molecules during deflagration can be hazardous for personnel. Specifically, reaction byproducts include hydrochloric acid, hydrofluoric acid, perfluoro-polymers, and carbonyl fluoride, which are known to be toxic.
Another deflagration suppressant is sodium bicarbonate which is injected into the combustible gas as solid particles. To generate and inject the particles, a solid containing the particles, such as a solid explosive composition, is typically combusted. The combustion vaporizes the sodium bicarbonate, which condenses in the ambient atmosphere as a plurality of small particles. The particles suppress the deflagration reaction by absorbing the heat and intercepting the free radicals generated by the deflagration.
Sodium bicarbonate has not been widely used as a deflagration suppressant since, for most applications, existing delivery systems are generally unable to deliver the particles to the combustible gas in sufficient time to suppress the deflagration reaction at an early stage. To be effective, deflagration suppression systems should deliver the suppressant rapidly to the combustible gas. The solid containing the particles often does not combust at a controlled rate, and is therefore unable to deliver the particles rapidly to the deflagration. Further many delivery systems are unable to disperse the particles uniformly throughout the area containing the combustible gas. Because deflagrations can occur in a variety of locations in a given area and propagate rapidly from the point of ignition, deflagration suppression systems should be able to rapidly and uniformly disperse the particles throughout the area.